For the first 2 billion years of the earth’s existence (4.5Ga to 2.5Ga) the atmosphere consisted largely of nitrogen and carbon dioxide (CO2), along with lesser amounts of methane, ammonia, Sulphur dioxide and water vapour. Little or no oxygen was present.
Evolving in this low oxygen environment, the first life appeared around 3.6Ga. A few hundred million years later primitive plants (cyanobacteria) had evolved the ability to split carbon dioxide into its constituent components of carbon and oxygen using solar radiation as an energy source. This biochemical process is known as photosynthesis. The plants used the carbon to make the materials of life, releasing the oxygen back to the atmosphere. As a consequence, the levels of oxygen in the atmosphere slowly rose. The solar energy was not lost: the greater part was stored by the plant in the chemical bonds of its organic compounds.
Excepting only the energy of radioactive elements, photosynthesis is the most efficient energy storage system that we know.
Between 2.5Ga and 1.8Ga, photosynthetic processes continued to evolve and become more efficient, leading to a dramatic increase in the levels of oxygen in the atmosphere. This is referred to as the Great Oxidation Event, or GOE (if one can consider a process lasting several hundred million years an event). Today the atmosphere contains around 20% oxygen.
The reverse process to photosynthesis is called respiration. In respiration, plants power their life activities by burning their stored carbohydrates in oxygen, releasing CO2 back to the atmosphere.
Iron makes up 10-15% (by weight) in basaltic rocks. The early crust was dominated by rocks of basaltic composition.
Iron has two valency states: ferrous iron, written Fe 2+ (or 2 electrons short of the full set) and ferric iron written Fe 3+ (3 electrons short). The metal thus has great capacity for combining with elements of negative charge, particularly oxygen, ever hungry for a positively charged valency partner. If only a small amount of oxygen is available, iron forms low-oxidation ferrous compounds. Where there is abundant oxygen, it forms high-oxidation ferric compounds. There is an important distinction between the two: ferrous iron compounds are generally soluble in water; ferric iron compounds are not.
In the low oxygen atmosphere of the first 2 billion years of earth history, chemical erosion of basaltic rocks at surface produced soluble ferrous iron compounds which, washed to the sea, slowly accumulated in the oceans of the world. Trillions upon trillions of tons of dissolved iron. During the GOE, over a relatively short geological span of a few hundred million years or so, that reservoir of ferrous iron became oxygenated to ferric iron which, being insoluble, settled as chemical sediment to the ocean floor to form thick iron rich sequences of great thickness and lateral extent. In later geological periods, chemical sediments were dominantly carbonates; in the late Archaean and early Proterozoic, it was ferric iron.
These distinctive iron rich sequences are known as Banded Iron Formations (BIF) and are found in all the preserved shield areas of the world. Brazil, North America, India, Africa, Russia and Western Australia. They are the source of ore for over 90% of our iron and steel. BIFs contain around 30% iron but can be locally upgraded, by natural processes, to 60-65% iron. Western Australia alone exports almost one billion tons per year of high-grade iron ore.
Banded iron formation in the Karinjini National Park, Hamersley Range, Western Australia. Image by the WA Tourist Department
But another process was at play during the GOE.
Atmospheric oxygen is chemically corrosive. The GOE supercharged erosion of all exposed land surfaces especially as back then there were no land plants to bind soils and constrain river channels. Beginning around 2.5Ga, increasing floods of sediments poured off the land to fill shallow basinal seas around continental margins with thick sequences of sandstone and siltstone. At the base of these sequences, interspersed with terrigenous clastic sediment, are Banded Iron Formations.
Abundant, cheap iron and steel support the skeletal frameworks of our civilisation. From bridges to buildings, ships to railroads, trucks to turbines to tanks, we owe our ready supply to the Great Oxidation Event, and the fossil remains of plant material with which to smelt it.